This is a K23 application for Dr. Julie Yabu, a transplant nephrologist at Stanford University, who proposes a novel, multidisciplinary approach to understanding the humoral immune response in sensitized kidney transplant recipients. This award will allow Dr. Yabu the resources, mentoring, and training to achieve the following career development goals: (1) to become an independent, translational clinical researcher in kidney transplantation; (2) to become an expert in the application of immunological methods to sensitized patients; and (3) to utilize biostatistical methods to understand the risk factors for sensitization. To achieve these goals, Dr. Yabu has assembled a mentoring team comprised of her sponsor and primary mentor, Dr. Glenn Chertow (expert in the clinical investigation of kidney disease) and three co-mentors: Dr. Mark Davis (renowned expert in immunology and developer of tetramer assays), Dr. Stephen Quake (pioneer in microfluidics and deep sequencing), and Dr. Paul J. Utz (expert in B-cell biology and translational research). Kidney transplantation is the most effective treatment for end-stage kidney disease. Sensitization, the formation of human leukocyte antigen (HLA) antibodies, remains a major barrier to successful kidney transplantation. Sensitized patients comprise 25% of the kidney transplant waiting list, but only 6.5% of sensitized patients are transplanted each year. Despite the implementation of desensitization strategies, many patients fail to respond for unknown reasons and rates of rejection and graft loss are high. Current progress in desensitization therapies is hindered by the lack of in-depth immune monitoring strategies to guide therapy and the absence of biomarkers beyond HLA antibodies, which may be late and unreliable markers of allograft injury. Using samples from a cohort of sensitized patients undergoing desensitization and transplantation, Dr. Yabu will use innovative techniques to determine longitudinal changes in B-cell immune profiles leading to HLA antibody production and correlate with rejection episodes and graft loss. Using data from the Stanford kidney transplant waiting list, Dr. Yabu will identify key risk factors associated with sensitization. Three specific aims are proposed: Aim 1: To determine the immune repertoire of B-cells in patients undergoing desensitization by deep sequencing B-cell receptors; Aim 2: To characterize the antigen specificity and intracellular signaling networks of B-cells that recognize HLA antigens in patients undergoing desensitization using CyTOF(R) mass spectrometry and MHC- peptide tetramer technology; and Aim 3: To identify clinical risk factors associated with sensitization by cross- examining the Stanford kidney transplant and USRDS databases. A detailed curriculum focusing on applied immunology, biostatistics and study design, scientific conferences, and professional development activities have been designed for Dr. Yabu's training. The planned research will form the basis for prospective studies using anti-B-cell therapies in sensitized kidney transplant recipients to be proposed in an R01 application before the end of the K23 award period.